SHANGHAI, Oct 12 (Reuters) - The top Chinese steelmaking city of Tangshan has ordered mills to cut output a month earlier than expected to meet tough 2017 air quality targets, according to a notice released by the municipal government and reviewed by Reuters.

Steel enterprises in Tangshan, located in heavily polluted Hebei province, must cut sintering output by 50 percent from Thursday, more than a month ahead of the original schedule, said the Oct. 11 notice, which Reuters received from three industry sources.

Sintering is a process where iron ore is heated into a mass as a precursor to making steel.

The notice said heavy smog was forecast to hit the city as early as Friday, forcing Tangshan to implement its "staggered production" plans a month early. Planned restrictions on outdoor construction activities will also be implemented beginning on Oct. 12.

The authenticity of the document was confirmed by an official with the Tangshan Iron and Steel Association on Thursday. He said Handan, another major steel producing city in Hebei, which surrounds Beijing, had issued a similar notice.

Handan city authorities told local steel mills to cut blast furnace production by 50 percent from Oct. 1 until March.

Tangshan is the latest city to launch a campaign to tackle toxic air ahead of the previous deadline of Nov. 15, when winter heating systems in China are switched on.

The city produces nearly 100 million tonnes of crude steel a year, more than the United States, and is routinely listed among China's 10 smoggiest cities. It vowed last month to shut ceramic and cement factories and restrict road freight activities during the heating season.

The final detailed plan is expected to be announced soon, according to the notice.

As part of a state "battleplan" to reduce air pollution in northern China this winter, Tangshan is under orders to cut emissions of hazardous particles known as particulate matter 2.5 (PM2.5) by 22 percent from October to March.

The central government announced in August that 28 cities in and around Hebei would be forced to cut PM2.5 by at least 15 percent during the winter months to meet politically crucial air quality targets for the region. (Reporting by David Stanway and Ruby Lian in SHANGHAI, Josephine Mason and Shu Zhang in BEIJING; Editing by Christian Schmollinger)